The process of electrodeposition is widely used commercially in processes such as electrorefining, electrowinning and electroplating. In commercial electrodeposition operations organic and/or inorganic addition agents are added directly to electrolytic solutions. The addition agents control uniformity of metal deposition on a cathode. When the addition agents are out of balance for proper electrodeposition, the metal deposit forms rough porous surfaces and nodules which encapsulate impurities contained in the electrolyte. Improper deposition typically greatly reduces the value of the product due to impurities mechanically imbedded in the rough cathode surface.
Copper electrorefineries around the world typically use a combination of several addition agents to control electrorefining. Addition agents used for electrorefining include animal glue, thiourea, lignin sulfonate, alkyl sulfonate and chloride ion. Positively charged addition agents such as animal glue are drawn by electrochemical forces to the negatively charged cathode. Positively charged addition agents are more strongly attracted to increased current density regions of peaks or nodules formed on a cathode. The increased concentration of addition agents on peaks or nodules slows down the metal electrodeposition and levelling takes place.
Advantageously, an addition agent such as glue is preferentially absorbed on the peak or nodule to form a resistance layer which locally increases over-potential and levelling on the cathode surface takes place. If excess addition agent is present, the addition agent adsorbs over an entire cathode surface which causes a loss of levelling effect. If insufficient addition agent is present, growth on peaks and nodules is not prevented and the peaks and nodules grow in an uncontrolled accelerated manner. Typical optimum concentration of addition agents is in the parts per million range. Unfortunately, concentrations of addition agents are very difficult to measure in a simple and accurate manner. Furthermore, several addition agents break down into multiple components and eventually lose levelling effect.
Typical copper electrorefinery addition agent systems are complicated and include a combination of three or more addition agents. As a result of high interactions between addition agents, levelling effects of new combinations of levelling agents are unpredictable. To evaluate an addition agent system time consuming laboratory or pilot plating experiments have been required. A typical experiment requires 7 to 14 days to complete. It would require about 5 to 10 years (without simultaneous experiments) to investigate every combination of a system of four addition agents each at four different concentrations.
T. Zak, in "Microlevelling During Electrolytic Deposition of Metals," Translation of the Institute of Metal Finishing, Vol. 49, (1971), pp. 220-26, discloses a laboratory set up designed for attempting to measure potential difference of cathodes. Laboratory equipment of Zak used cathodes having alternating plates insulated and spaced 0.02 mm apart and every second cathode was either 0.01 or 0.02 mm closer to an anode. The set up of Zak was unable to record a difference in potential between protruding and recessed electrode depending upon any addition agent used. In contrast, several techniques have been successfully developed to monitor addition agent concentrations in electrolyte. Langner et al, in U.S. Pat. No. 4,834,842, describe a technique of measuring effectiveness of addition agents by measuring kinetics of cathode polarization under predetermined conditions. Other techniques described in the literature have measured cathode polarization in an attempt to optimize plating conditions. These cathode polarization techniques are not capable of measuring the ability of an addition agent or a combination of addition agents to alter cathode levelling.
It is an object of this invention to provide an apparatus and method for evaluating the ability of an addition agent to improve cathode surface during electrodeposition.
It is a further object of this invention to provide a quick and effective method for evaluating addition agents and their combination for cathode levelling.
It is a further object of this invention to provide a method for controlling levelling power of electrolytes to prevent the formation of a rough nodulated and contaminated surface by adjusting the addition agents concentration.